Image processing apparatus, image processing method, and computer-readable encoding medium encoded with a computer program

ABSTRACT

An image processing apparatus, including a color conversion part configured to conduct a color conversion of image data. The color conversion part extends an information amount of an output value in the color conversion based on a first minimum information amount possible to depict a gradation in a lattice point interval of an input value in a look up table used for the color conversion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an image processingapparatus, an image processing method, and a computer-readable encodingmedium encoded with a computer program, and more particularly to theimage processing apparatus, the image processing method, and thecomputer-readable encoding medium encoded with a computer program, inwhich a color conversion or a density correction is conducted.

2. Description of the Related Art

FIG. 1 is a block diagram illustrating an example of image processesconducted by a printer driver, a controller of a printer, or a like.FIG. 1 illustrates that the image processes including a colorconversion, a γ-correction, a halftoning process are conducted withrespect to image data in which each gradation of an RGB (Red, Green, andBlue) is 8 bits for each pixel.

In the color conversion, a color space in which the image data are inputis converted into another color space of a CMYK (Cyan, Magenta, Yellow,and Black) for an ink color. As a result, the image data in which eachgradation of the CMYK is 8 bits for each pixel are output. In theγ-correction, a density correction is conducted, and the image data inwhich each gradation of the CMYK is 8 bits are output. In the halftoningprocess, a pseudo-continuous tone process is conducted by a dithermethod, a diffusion method, or a like, a gradation (8 bits) of the imagedata is converted into another gradation (for example, 2 bits)corresponding to a printer.

As illustrated in FIG. 1, there is a problem in that a gradation numberof the output image is substantially deteriorated, if an informationamount for depicting a gradation of an output image is lower than aninformation amount (8 bits) for depicting a gradation of an input imagein a color conversion.

FIG. 2 is a diagram for explaining deterioration of the gradation numberin the color conversion. FIG. 2 illustrates that input values (inputimage) are converted into output values (output image) by a colorconversion using an LUT (Look Up Table) 510. In FIG. 2, a value of K(black) is “32” in an output value (lattice point value) with respect toan input value (lattice point) (0, 0, 0). Also, the value of K (black)is “28” in the output value with respect to an input value (0, 0, 16).Output values with respect to input values among lattice points areinterpolated by conducting an interpolation calculation process such asa cubic interpolation, a triangular prism interpolation, a tetrahedroninterpolation, or a like. Calculation results for K (black) by using theinterpolation calculation process are exemplified as interpolationvalues in FIG. 2. However, if the information amount for depicting thegradation of the output value is the same as that of the input value, adifference with a value less than one (after the decimal point) can notbe expressed. Accordingly, values defined as actual output values inFIG. 2 are applied as output values. As a result, 16 levels (0 through16) of the gradation in the input values are reduced to 4 levels (32through 28) of the gradation in the output values. As described above,if the information amount for depicting the gradation of the outputimage is fewer than the information amount of the input image, aresolution of an output side becomes lower than a lattice pointinterval, and the gradation number is substantially degraded.

Moreover, since a γ-correction parameter is generally curvilinear inshape, there is a problem in that when the information amount fordepicting the gradation of the output image in the density correction ofthe γ-correction or a like is less than the information amount (8 bitsin this example) for depicting the gradation of the input image, thegradation number of the output image is substantially degraded.

FIG. 3 is a diagram for explaining deterioration of the gradation numberin the density correction. In FIG. 3, a table 520 shows input values andlogical values. Also, the table 530 shows actual output values. As shownin table 530, if the information amount for depicting the gradation ofthe output image is less than the information amount for depicting theinput image, it is not possible to depict a variance in values less thanone in an ideal output value. As a result, the gradation number of theoutput value is deteriorated.

The deterioration of the gradation number described above causesdiscontinuity in a gradation image and causes a degraded image which issufficiently recognizable by human eyes.

SUMMARY OF THE INVENTION

The present invention solves or reduces one or more of the aboveproblems.

In an aspect of this disclosure, there is provided an image processingapparatus, comprising a color conversion part configured to conduct acolor conversion of image data, wherein the color conversion partextends an information amount of an output value in the color conversionbased on a first minimum information amount possible to depict agradation in a lattice point interval of an input value in a look uptable used for the color conversion.

In the image processing apparatus, it is possible to properly suppress aconsumption amount of a resource used to prevent deterioration of animage quality in an image process.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating an example of image processesconducted by a printer driver, a controller of a printer, or a like;

FIG. 2 is a diagram for explaining deterioration of a gradation numberin a color conversion;

FIG. 3 is a diagram for explaining deterioration of a gradation numberin a density correction;

FIG. 4 is a block diagram illustrating a configuration example of animage process system according to an embodiment of the present invention

FIG. 5 is a block diagram illustrating a functional configurationexample of an image process part;

FIG. 6 is a diagram illustrating a hardware configuration example of aPC according to the embodiment of the present invention;

FIG. 7 is a flowchart for explaining process steps conducted by a colorconversion part;

FIG. 8 is a diagram illustrating a configuration example of a colorconversion table;

FIG. 9 is a diagram illustrating an example of an output value in a caseof extending an information amount of a gradation of the output image;

FIG. 10 is a flowchart for explaining process steps conducted by aγ-correction part;

FIG. 11 is a diagram illustrating a configuration example of aγ-correction table; and

FIG. 12 is a diagram illustrating an example of implementing the imageprocess part in the printer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, an embodiment of the present invention will bedescribed with reference to the accompanying drawings. FIG. 4 is a blockdiagram illustrating a configuration example of an image process systemaccording to the embodiment of the present invention. In FIG. 4, animage process system 1 includes a PC (Personal Computer) 10 and aprinter 20 which are connected through a network.

The PC 10 includes an input part 11, an image process part 12, and alike. The input part 11 inputs image data subject to be printed, to theimage process part 12. The image process part 12 conducts an imageprocess for converting image data to be input into image data suitablefor the printer 20 to print out. That is, in FIG. 4, the PC 10corresponds to an image processing apparatus. The image process part 12is generally embedded in software which is called a printer driver.

The printer 20 includes an output part 21. The output part 21 receivesimage data to which an image process is conducted by the image processpart 12 of the PC 10, and controls a process which makes the printer 20print out the image data.

The image process part 12 of the PC 10 will be described in detail. FIG.5 is a block diagram illustrating a functional configuration example ofthe image process part 12. In FIG. 5, the image process part 12 includesa color conversion part 121, a γ-correction part 122, a halftoningprocess part 123, a color conversion table 124, a γ-correction table125, and a like.

The color conversion part 121 converts a color space of the image datato be input into the CMYK used for an ink color. The color conversionpart 121 appropriately changes (increases) the information amount fordepicting gradation of output image data, with respect to theinformation amount for depicting gradation of the image data to be inputwhen conducting the color conversion. In FIG. 5, image data in which theinformation amount of the gradation is 8 bits are input to the colorconversion part 121, and image data in which the information amount ofthe gradation is extended to x bits are output. The color conversionpart 121 prevents deterioration of a gradation number substantial for anoutput image in which the color conversion is conducted, by increasingthe information amount of the gradation of the output image.

The γ-correction part 122 conducts the density correction (γ-correction)with respect to the image data in which the color conversion isconducted by the color conversion part 121. The γ-correction part 122approximately changes (increases) the information amount of thegradation of the output image with respect to the information amount fordepicting the gradation of the image data to be input, when conductingthe density correction. In FIG. 5, image data in which the informationof the gradation is x bits is input to the γ-correction part 122, imagedata in which the information amount of the gradation is extended to ybits. The γ-correction part 122 prevents deterioration of the gradationnumber substantial to the output image, which is caused by the densitycorrection, by increasing the information amount of the gradation of theoutput image.

The halftoning process part 123 conducts a pseudo-continuous toneprocess by a dither method, a diffusion method, or a like, and convertsthe gradation number of the image data into another gradation number(for example, 2 bits) corresponding to the printer 20.

The color conversion table 124 is an LUT (Look Up Table) used for thecolor conversion performed by the color conversion part 121. Theγ-correction table 125 is another LUT (Look Up Table) used for thedensity correction performed by the γ-correction part 122.

FIG. 6 is a diagram illustrating a hardware configuration example of thePC 10 according to the embodiment of the present invention. The PC 10 inFIG. 6 includes a driver unit 100, an auxiliary storage unit 102, amemory unit 103, a CPU (Central Processing Unit) 104, an interface unit105, a display unit 106, and an input unit 107, which are mutuallyconnected through a bus B.

A program realizing processes of the PC 10 is provided by a recordingmedium 101 such as a CD-ROM (Compact Disk Read Only Memory). When therecording medium 101 recording the program is set to the driver unit100, the program is installed to the auxiliary storage unit 102 from therecording medium 101 to the driver unit 100. The auxiliary storage unit102 stores the installed program, and also stores necessary files, data,and a like. For example, the color conversion table 124 and theγ-correction table 125 are stored in the auxiliary storage unit 102.

When an instruction is made to execute the program, the memory unit 103reads out the program from the auxiliary storage unit 102. The CPU 104realizes functions concerning the PC 10 in accordance with the programstored in the memory unit 103. The interface unit 105 is used as aninterface to connect to the network. The display unit 106 displays a GUI(Graphical User Interface) created by the program. The input unit 107includes a keyboard, a mouse, and a like, and is used to input variousoperation instructions.

It is not necessary to always install the program from the recordingmedium 101. The program may be downloaded from another computer throughthe network.

In the following, process steps of the image process part 12 will bedescribed. FIG. 7 is a flowchart for explaining the process stepsconducted by the color conversion part 121.

The image data input by the input part 11 is accepted (step S101). It isassumed that the image data, in which the information amount fordepicting each gradation of RGB (Red, Green, and Black) is 8 bits foreach pixel, is input. Subsequently, the color conversion table 124 isreferred to (step S102).

FIG. 8 is a diagram illustrating a configuration example of the colorconversion table 124. As illustrated in FIG. 8, only a correspondencebetween RGB values for an input value (a lattice point) and CMYK valuesfor an output value (lattice point value) is registered in the colorconversion table 124. In FIG. 8, the input value (0, 0, 0) correspondsto the output value (64, 64, 64, 32), and the input value (0, 0, 16)corresponds to the output value (80, 80, 96, 28). In the colorconversion table 124, both the input value and the output value aredefined with an information amount (8 bits) as the same as theinformation amount of the gradation of the input image.

Subsequently, in order to retain the gradation number (input resolution)of the input image in the output image, the information amount (outputresolution) for depicting the gradation of the output image iscalculated (step S103). In this embodiment, the information amount fordepicting the gradation of the output image is calculated by using thefollowing formula (1):n+┌ log₂s┐ bit  (1)

-   -   n: the number of bits of output value in the color conversion        table    -   s: lattice point interval, ┌ ┐: round up

In a case in FIG. 8, n=8 and s=16. Accordingly, the information amountfor depicting the gradation of the output image of the color conversionis 12 bits as follows:8+┌ log₂16┐=8+4=12 bit

The formula (1) is to extend the information amount of the output valuein the color conversion table 124 by using a minimum information amountpossible to depict the gradation in a lattice point interval of theinput value (an interval among the input values) in the color conversiontable 124. That is, it is assumed that there is a case in that aninterval among the output values is a minimum value (that is, when theinterval is “1”) with respect to the lattice point interval (an intervalamong input values). Even in this case of the minimum value, the formula(1) calculates a minimum information amount essential not to deterioratethe gradation number in the output image. In detail, in this embodiment,the information amount of the gradation of the output image is extendedto 4 bits with respect to the input image. It is possible to depict 16levels of the gradation if extending to 4 bits. Accordingly, for thesmallest range of the interval of the output values with respect to thelattice point interval, it is possible to depict the gradation with 16levels. Thus, it is possible to prevent the deterioration of thegradation number in for the entire range. Hereinafter, the informationamount extended with respect to the information amount of the gradationof the output value of the color conversion table 124 is called an“extended information amount”.

A value calculated by the formula (1) beforehand may be stored as theinformation amount for depicting the gradation of the output image inthe auxiliary storage unit 102. In this case, the color conversion part121 is not required to calculate based on the formula (1) in the stepS103, but may acquire the value stored in the auxiliary storage unit102.

Subsequently, a conversion value (the output value) of the colorconversion is output by using the color conversion table 124 (stepS104). In this case, the color conversion part 121 calculates aninterpolation value by the interpolation calculation process such as thecubic interpolation, the triangular prism interpolation, or thetetrahedron interpolation for the output value with respect to the inputvalue among lattice points, and calculates a value (output value) byconducting the following calculation with respect to the interpolationvalue.[IPV×EGN]÷EGN

In this calculation, IPV denotes the interpolation value, EGN denotesthe gradation number which can be depicted with the extended informationamount, and [ ] denotes rounding down a decimal point. In this case,since the extended information amount is 4 bits, the gradation numberpossible to depict with the extended information amount becomes 16levels. By applying the above-described calculation, it is possible toacquire the output image retaining the gradation number of the inputimage.

FIG. 9 is a diagram illustrating an example of the output value in acase of extending the information amount of the gradation of the outputimage. In FIG. 9, illustrates that the interpolation values and theoutput values for a range of the input values (0, 0, 0) through (0, 0,15). For the sake of convenience, only values for K (black) areexemplified for the interpolation values and the output values. Asdepicted in FIG. 9, the information amount for depicting the gradationof the output value is extended to 12 bits. Accordingly, for a range inwhich only 4 levels are acquired in conventional technologies, it ispossible to depict the gradation with 16 levels and prevent thedeterioration of the gradation number.

In the foregoing, the color conversion from RGB to CMYK is described.Also, other color conversions can be conducted in the same manner, fromRGB to RGB, from RGB to CMY (Cyan, Magenta, and Yellow), from RGB toCMYKR (Cyan, Magenta, Yellow, Black, and Red), or a like.

The image data to which the color conversion is conducted are input tothe γ-correction part 122. FIG. 10 is a flowchart for explaining processsteps conducted by the γ-correction part 122.

The image data to which the color conversion is conducted by the colorconversion part 121 are input and accepted (step S201). It is assumedthat information amount for depicting each gradation of CMYK (Cyan,Magenta, Yellow, and Black) is 12 bits for each pixel is input.Subsequently, the γ-correction table 125 is referred to (step S202).

FIG. 11 is a diagram illustrating a configuration example of theγ-correction table. As illustrated in FIG. 11, the input value of 12bits and the output value of 14 bits are registered in the γ-correctiontable 125 to correspond to each other by one to one. The output value of12 bits and the logical value do not form the γ-correction table 125 butare exemplified as a reference. The logical value is an output valuecalculated based on a gamma function. It should be noted that numeralvalues are 16-fold values.

As illustrated in FIG. 11, the information amount (bit number) of theoutput value is extended with respect to the input value in theγ-correction table 125 according to the embodiment of the presentinvention. After extended, the information amount is calculated based onthe following formula (2):n+┌ log₂k┐ bit  (2)

-   -   n: the number of bits of input value    -   k: reciprocal number of a minimum change rate, ┌ ┐: round up

In this case, n=12. Also, a reciprocal number k of a minimum change rateof the output value indicates a change amount of the gradation(gradation number) of the input value with respect to a change of onelevel in the gradation of the output value when the information amountof the input value is the same as the information amount of the outputvalue. In FIG. 11, one level in the gradation of the output value of 12bits corresponds to 4 levels as the gradation number of the input value.Accordingly, in this embodiment, the information amount for depictingthe gradation of the output image of the γ-correction is 14 bits asexpressed below.12+┌ log₂4┐=12+2=14 bit

That is, the formula (2) extends the information amount of the outputvalue with respect to the information amount of the input value by usinga second minimum information amount possible to depict a change amountof the gradation of the input value with respect to a change for onelevel in the gradation of the output value when the information amountof the input value is the same as the information amount of the outputvalue. In detail, in this embodiment, the information amount of thegradation of the output image increases 2 bits more than the inputimage. It is possible to depict the gradation of 4 levels by using 2bits. Accordingly, it is possible to depict a change being less than onein the output value during the input value changes four levels. Thus,each output value can be different with respect to all input values.Hereinafter, an additional information amount (2 bits) to theinformation amount of the input image is called an “extended informationamount”. It should be noted that the output value with respect to eachinput value (14 bits) may be acquired based on the followingcalculation:[LV×EIA]÷EIA

In this calculation, LV denotes the logical value, EIA denotes thegradation number possible to depict the extended information amount, and[ ] denotes rounding down a decimal point. In this case, since theextended information amount is 2 bits, the gradation number possible todepict with the extended information amount becomes 4 levels. Byapplying the above-described calculation, it is possible to acquire theoutput image retaining the gradation number of the input image.

Subsequently, the output value with respect to the input value is outputbased on the γ-correction table 125 created as described above (stepS203). Accordingly, 12 bits are extended to 14 bits and are output. As aresult, a γ-correction result is output without deteriorating thegradation number of the input image.

Values registered as the input values in the γ-correction table 125beforehand are not limited to 12 bits but may be 8 bits. It should benoted that the output values are based on the extended informationamount (for example, 14 bits). In this case, it is possible tocorrespond to the input value of 12 bits by extending the value of 8bits registered in the γ-correction table 125 by using a linerinterpolation.

As described above, by the image process part 12 in this embodiment, theinformation amount of the output value is extended by using the minimuminformation amount possible to depict the gradation in the lattice pointinterval of the input value in the color conversion table 124 in thecolor conversion. Accordingly, the deterioration of the gradation numberin the output image can be prevented, and an increase of an excessiveconsumption amount of a memory can be suppressed.

Moreover, in the density correction, the information amount of theoutput value is extended based on the minimum information amountpossible to depict a change amount of the gradation of the input valuewith respect to a change of one level in the gradation of the outputvalue when the information amount of the input value is the same as theinformation amount of the output value. Accordingly, the deteriorationof the gradation number (the deterioration of an image quality) in theoutput image can be prevented, and the increase of the excessiveconsumption amount of the memory can be suppressed.

This embodiment exemplifies an extension of the information amount ofthe output value in both the color conversion and the densitycorrection. Alternatively, with respect to either one of the colorconversion and the density correction, the information amount of theoutput value may be extended.

Also, the following formula (3) may be applied, instead of the formula(1).┌a×(n+log₂s)┐ bit  (3)

-   -   n: the number of bits of output value in the color conversion        table    -   a: correction coefficient (0.7-1.3)    -   s: lattice point interval, ┌ ┐: round up

Also, the following formula (4) may be applied, instead of the formula(2).┌a×(n+log₂k)┐ bit  (4)

-   -   n: the number of bits of input value    -   a: correction coefficient (0.7-1.3)    -   k: reciprocal number of a minimum change rate, ┌ ┐: round up

In the formulae (3) and (4), a correction coefficient “a” is multipliedto the formulae (1) and (2), respectively. For example, the correctioncoefficient “a” may be increased or decreased depending of areproducible color gamut by combining a print sheet and an ink. Indetail, when the correction coefficient a=1.0 is applied in a case ofusing a regular paper for a inkjet, since a sheet having a narrow colorgamut, which is typified by the regular paper such as a PPC (Plain PaperCopier) sheet or a like, decreases a resolution for a necessarygradation, the correction coefficient a is set to be less than 1.0 (forexample, 0.8). On the other hand, a sheet having a wider color gamut,which is typified by a glossy paper for a picture, increases theresolution for the necessary gradation, the correction coefficient “a”is set to be greater than 1.0 (for example, 1.2). In each methoddescribed above, it is possible to obtain the information amount (bitnumber) which unnecessarily consumes a resource. Alternatively, thecorrection coefficient “a” may be increased and decreased based onwhether an ink is a pigment or a dye, or which special color such asred, blue, or a like is used.

Moreover, the image process part 12 in the print system 1 may beimplemented in the printer 20. FIG. 12 is a diagram illustrating anexample of implementing the image process part 12 in the printer 20. Inan image process system 1-2 illustrated in FIG. 12, the image processpart 12 is implemented in the printer 20. In this case, the printer 20corresponds to an image processing apparatus.

According to the present invention, there are provided an imageprocessing apparatus, an image processing method, and acomputer-readable encoding medium encoded with a computer program, inwhich it is possible to suppress an amount of a resource used to preventthe deterioration of the image quality in the image process.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the invention.

An image processing apparatus according to the present invention mayinclude a density correction part configured to conduct a densitycorrection of image data, wherein the density correction part extends aninformation amount of an output value in the density correction based ona minimum information amount possible to depict a change amount of agradation of an input value with respect to a change of one level in agradation of the output value when an information amount of the inputvalue is the same as the information amount of the output value.

In an image processing method according to the present invention, animage processing apparatus may conduct a density correction of imagedata, wherein in conducting the density correction, an informationamount of an output value in the density correction is extended based ona minimum information amount possible to depict a change amount of agradation of an input value with respect to a change of one level in agradation of the output value when an information amount of the inputvalue is the same as the information amount of the output value.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the invention.

The present application is based on the Japanese Priority PatentApplication No. 2007-314705 filed Dec. 5, 2007, the entire contents ofwhich are hereby incorporated by reference.

1. An image processing apparatus, comprising a color conversion partconfigured to conduct a color conversion of image data, wherein thecolor conversion part extends an information amount of an output valuein the color conversion based on a first minimum information amountpossible to depict a gradation in a lattice point interval of an inputvalue in a look up table used for the color conversion.
 2. The imageprocessing apparatus as claimed in claim 1, wherein the color conversionpart sets an information amount resulted from adding the first minimuminformation amount to an information amount of the output value in thelook up table, as an information amount of an output value in the colorconversion.
 3. The image processing apparatus as claimed in claim 1,further comprising a density correction part configured to conduct adensity correction of the image data to which a color space is convertedby the color conversion part, wherein the density correction partextends an information amount of an output value in the densitycorrection, based on a second minimum information amount possible todepict a change amount of a gradation of an input value with respect toa change of one level in a gradation of the output value when aninformation amount of the input value is the same as the informationamount of the output value.
 4. The image processing apparatus as claimedin claim 3, wherein the density correction part sets an informationamount resulted from adding the second minimum information amount to aninformation amount for depicting a gradation of image data prior to thedensity correction, as an information amount of an output value in thedensity correction.
 5. An image processing method conducted in an imageprocessing apparatus, comprising conducting a color conversion of imagedata, wherein in conducting the color conversion, an information amountof an output value in the color conversion is extended based on a firstminimum information amount possible to depict a gradation in a latticepoint interval of the input value in a look up table used for the colorconversion.
 6. The image processing method as claimed in claim 5,wherein in conducting the color conversion, an information amountresulted from adding the first minimum information amount to aninformation amount of the output value in the look up table is set as aninformation amount of an output value in the color conversion.
 7. Theimage processing method as claimed in claim 5, further comprisingconducting a density correction of image data in which a color space isconverted in conducting the color conversion, wherein in conducting thedensity correction, an information amount of an output value in thedensity correction is extended based on a second minimum informationamount possible to depict a change amount of a gradation of an inputvalue with respect to a change amount of one level in a gradation of theoutput value when an information amount of the input value is the sameas the information amount of the output value.
 8. The image processingmethod as claimed in claim 7, wherein in conducting the densitycorrection, an information amount resulted from adding the secondminimum information amount to an information amount for depicting agradation of image data prior to the density correction is set as aninformation amount of an output value in the density correction.
 9. Anon-transitory computer-readable encoding medium encoded with a computerprogram, said computer program comprising codes for conducting a colorconversion of image data, wherein in conducting the color conversion, aninformation amount of an output value in the color conversion isextended based on a first minimum information amount possible to depicta gradation in a lattice point interval of an input value in a look uptable used for the color conversion.